Gamma correction circuit for correcting video data obtained by photoelectric transfer and having a non-linear characteristic

ABSTRACT

A gamma correction circuit including a first look-up table in which minimum values of differences between output levels of a gamma correction curve and output levels of a straight line in respective sections formed by dividing a range from a minimum input level to a maximum input level into sixteen are stored. Furthermore, data of differences between the output levels of the gamma correction curve and levels obtained by adding the minimum values to the output levels of the straight line in the respective sections are stored in respective second look-up tables of sixteen. Then, one of the minimum values is read-out from the first look-up table according to upper four bits of input video data, and one of the second look-up tables is selectively enabled by the upper four bits, and data stored in the selected second look-up table is read-out in accordance with lower six bits of the input video data. Outputs from the first and second look-up tables are added to each other so as to output gamma correction data which is then added to the input video data, whereby a gamma characteristic of the input video data is corrected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gamma correction circuit. Morespecifically, the present invention relates to a gamma correctioncircuit in which video data obtained by photoelectric transfer andhaving a nonlinear characteristic is corrected in accordance with agamma correction curve.

2. Description of the Related Art

In a conventional gamma correction circuit 1 as shown in FIG. 5, data ofdifferences between output levels y =γ(x) of a gamma correction curveand output levels y=x of video data shown in FIG. 4 are stored in alook-up table 2 constituted by a ROM, and one of the data correspondingto input video data is read-out from the look-up table 2. Then, the dataread-out is multiplied by a gamma correction coefficient γ by amultiplier 3, and upper ten bits of multiplication result data and thevideo data are added to each other by an adder 4. Therefore, the gammacorrection characteristic can be changed in accordance with the gammacorrection coefficient γ and the video data can be corrected by thegamma correction characteristic.

However, in such a prior art, since the data of the differences betweenthe output levels of the gamma correction curve and the output levels ofthe video data are written into the look-up table 2, it is needed toincrease a memory capacity of the look-up table 2 in accordance with thegamma correction curve, i.e. an exponent of power.

SUMMARY OF THE INVENTION

Therefore, a principal object of the present invention is to provide anovel gamma correction circuit.

Another object of the present invention is to provide a gamma correctioncircuit capable of reducing a memory capacity of a look-up table.

According to the present invention, a gamma correction circuitcomprises: a first look-up table which stores a minimum value ofdifferences between output levels of a gamma correction curve and outputlevels of a straight line indicative of an input and outputcharacteristic of an input video signal for each of sections formed bydividing a range from a minimum input level to a maximum input level; aplurality of second look-up tables in each of which data of differencesbetween the output levels of the gamma correction curve and levelsobtained by adding the minimum values to the output levels of thestraight line for each of the sections are stored; selection means forselecting one of the second look-up tables in accordance with a level ofthe input video signal; and a first addition means which adds theminimum value read-out from the first look-up table in accordance withthe level of the input video signal and the data read-out from thesecond look-up table selected by the selection means.

The range from the minimum input level to the maximum input level isdivided into sixteen sections, for example, and the minimum value of thedifferences between the output levels of the gamma correction curve andthe output levels of the straight line is stored in advance in the firstlook-up table for each of the sections. In addition, the straight lineis for indicating an input and output characteristic of the videosignal, for example, and the video signal is converted into the videodata of ten bits, for example. Upper four bits of the video data areapplied to the first look-up table as its address data, for example, andtherefore, the minimum value corresponding to the level of the videodata is read-out from the first look-up table. In contrast, the data ofthe differences between the output levels of the gamma correction curveand the data obtained by adding the minimum value to the output levelsof the straight line are stored in advance in each of the second look-uptables of sixteen, for example. Lower six bits of the video data areapplied to the respective second look-up tables as addresses therefor,and therefore, data corresponding to the level of the video signal,i.e., the lower six bits of the video data are read-out to be applied toa selection circuit. The upper four bits of the video data are alsoapplied to the selection circuit as selection data, and therefore, thedata corresponding to the level of the video signal, i.e., the upperfour bits of the video data is selected. That is, one of the secondlook-up tables is selected. Thereafter, the minimum value read-out fromthe first look-up table and the data read-out from the selected secondlook-up table are added to each other by an adder to obtain gammacorrection data. The gamma correction data is added to the input videodata, whereby the gamma characteristic of the input video data iscorrected.

According to the present invention, since the data of the second look-uptables are optimized, it is possible to reduce a total memory capacityof the first and second look-up tables.

The above described objects and other objects, features, aspects andadvantages of the present invention will become more apparent from thefollowing detailed description of the present invention when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing one embodiment according to thepresent invention;

FIG. 2 is an illustrative view showing a first look-up table;

FIG. 3 is an illustrative view showing a second look-up tables;

FIG. 4 is a graph showing a gamma correction curve and an input andoutput characteristic of video data; and

FIG. 5 is a block diagram of related art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A gamma correction circuit 10 of this embodiment shown in FIG. 1includes an input terminal 14 which receives video data of ten bits, forexample. Upper four bits included in the video data are applied to aselection circuit 18 and a first look-up table 20 as control data andaddress data D1, respectively, and lower six bits of the video data areapplied to second look-up tables 12 of sixteen as address data D2.Therefore, the first look-up table is read-out by the address D1, i.e.the upper four bits of the video data, and the second look-up tables areread-out by the address D2, i.e. the lower six bits of the video data.Furthermore, the video data itself is applied to an adder 28.

In the first look-up table 20, minimum value data b(D1) are stored incorrespondence with respective address data D1 as shown in FIG. 2. Morespecifically, a range from a minimum input level to a maximum inputlevel of the video data, shown in FIG. 4, is divided into sixteensections. Then, with referring to FIG. 4, in a case of the section fromx₁ to x₂, a difference between an output level y=γ(x₂) of the gammacoffection curve and an output level y=x₂ of the video data becomesminimum out of differences between output levels y=γ(x) and outputlevels y=x. Such the minimum value data b(D1) for each of the sectionsis stored in each address represented by the address data D1 from "0" to"16"of the first look-up table 20.

Furthermore, as shown in FIG. 3, data c(D2) are stored in the respectivesecond look-up tables 12 in correspondence with the address data D2 from"0" to "63" for each of the sections. More specifically, range of 0≦x≦1023 is equally divided into the sixteen sections, and differencesbetween the output levels y=γ (x) of the gamma correction curve andlevels y=x +b(D1) obtained by adding the minimum value data b(D1) to theoutput levels y=x are stored in each of the second look-up tables 12 asthe data c(D2). Accordingly, one of the data c(D2) is indicated by afollowing equation (1).

    c(D2)=γ(x)-(x +b(D1))                                (1)

The data c(D2) corresponding to the address data D2 are outputted fromthe respective second look-up tables 12. However, since bit lengths ofthe respective data c(D2) are different from each other as understoodfrom FIG. 3, output bit lengths of the respective second look-up tables12 become different from each other.

Accordingly, dummy bits are added to upper portions of the respectivedata c(D2) which are read-out by bit expansion circuits 16 correspondingto the respective second look-up tables 12, whereby the bit lengths ofthe data c(D2) are uniformed to eight bits. The selection circuit 18selects one of the data c(D2) in accordance with the control data. Theselected data c(D2) is added with a dummy bit by a bit expansion circuit22, and then applied to an adder 24.

In contrast, the minimum value data b(D1) corresponding to the addressdata D1 is read-out from the first look-up table 20, and the minimumvalue data b(D1) is added to the data c(D2) from the bit expansioncircuit 22 by the adder 24. Then, addition result data b(D1) +c(D2) ismultiplied by a gamma correction coefficient γ of seven bits by amultiplier 26. Lower six bits of multiplication result data{b(D1)+c(D2)}×γ of sixteen bits are omitted by an omission circuit (notshown), and only upper ten bits of the multiplication result data areapplied to an adder 28. Thus, by omitting the lower six bits, it ispossible to multiple the addition result data b(D1) +c(D2) by a decimalnumber. That is, in a case where "128" is applied to the multiplier 26as the gamma correction coefficient γ the addition result value isdoubled; however, in a case where "32" is applied to the multiplier 26as the gamma correction coefficient γ the addition result value ishalved.

The multiplication result data of ten bits are added to the video dataof ten bits by the adder 28, and then, upper one bit is clipped by anoverflow clip circuit 30. Accordingly, video data which is indicated bya following equation (2) and has been subjected to the gamma correctionis outputted from the overflow clip circuit 30.

    y=x +{b(D1)+c(D2)}×γ                           (2)

Thus, since the data stored in the first look-up table 20 and the secondlook-up tables 12 are optimized, it is possible to drastically reduce atotal memory capacity in comparison with the prior art. Morespecifically, though a memory capacity of 9216 bits (=2¹⁰ ×9) wasnecessary for the look-up table 2 of the gamma correction circuit 1shown in FIG. 5, the total memory capacity of the first look-up table 20and the second look-up tables 12 of this embodiment becomes 5328 bits asindicated by a following equation (3), and it is possible to reduce thememory capacity about 42% in comparison with the prior art look-up table2.

    16×9+64×(8+6×4+5×8+4+3+2)=5328 bits (3)

In addition, the selection circuit 18 and the adder 24 are necessary forthe gamma correction circuit 10 of this embodiment; however, since theselection circuit can be constituted by gates of about fifty, and theadder 24 can be constituted by gates of about 300, there is not a bigproblem even if the above described embodiment is to be incorporatedinto an IC.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A gamma correction circuit, comprising:a firstlook-up table which stores a minimum value of differences between outputlevels of a gamma correction curve and output levels of a straight lineindicative of an input and output characteristic of an input videosignal for each of sections formed by dividing a range from a minimuminput level to a maximum input level; a plurality of second look-uptables in each of which data of the differences between the outputlevels of the gamma correction curve and levels obtained by addingminimum values to the output levels of the straight line for each of thesections are stored; selection means for selecting one of the secondlook-up tables in accordance with a level of the input video signal; anda first addition means which adds the minimum value read-out from thefirst look-up table in accordance with the level of the input videosignal and the data read-out from said one of the second look-up tablesselected by the selection means.
 2. A gamma correction circuit accordingto claim 1, wherein the video signal is digital video data of apredetermined number of bits, and the level is predetermined upper bitsof the video data, and said one of the second look-up tables is read-outby predetermined lower bits of the video data.
 3. A gamma correctioncircuit according to claim 2, further comprising a second addition meansfor adding an output of said first addition means and the video data toeach other.
 4. A gamma correction circuit according to claim 1, furthercomprising; uniforming means which uniforms bit lengths of the dataoutputted from the respective second look-up tables.
 5. A gammacorrection circuit according to claim 4, wherein the uniforming meansincludes a plurality of bit expansion means corresponding to therespective second look-up tables.